Recently, a secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices. Also, the secondary battery has attracted considerable attention as a power source for electric vehicles (EV) and hybrid electric vehicles (HEV), which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuel.
Small-sized mobile devices use one or several small-sized cells for each device. On the other hand, medium- or large-sized devices, such as vehicles, use a medium- or large-sized battery pack having a plurality of battery cells electrically connected with each other because high output and large capacity is necessary for the medium- or large-sized devices.
Preferably, the medium- or large-sized battery pack is manufactured with small size and small weight if possible. For this reason, a rectangular battery or a pouch-shaped battery, which can be stacked with high integration and has a small weight to capacity ratio, is usually used as a battery cell of the medium- or large-sized battery pack. Especially, much interest is currently generated in the pouch-shaped battery, which uses an aluminum laminate sheet as a battery case.
FIG. 1 is a perspective view typically illustrating a conventional representative pouch-shaped battery. The pouch-shaped battery 10 shown in FIG. 1 is constructed in a structure in which two electrode leads 11 and 12 protrude from the upper end and the lower end of a battery cell 13, respectively, while the electrode leads 11 and 12 are opposite to each other. A battery case 14 comprises upper and lower battery case parts. That is, the battery case 14 is a two-unit case. An electrode assembly (not shown) is received in a receiving part 15, which is defined between the upper and lower battery case parts of the battery case 14. The opposite sides 14a and upper and lower ends 14b and 14c, which are contact regions of the upper and lower battery case parts of the battery case 14, are bonded to each other, whereby the pouch-shaped battery 10 is manufactured. The battery case 14 is constructed in a laminate structure of a resin layer/a metal film layer/a resin layer. Consequently, it is possible to bond the opposite sides 14a and upper and lower ends 14b and 14c of the upper and lower battery case parts of the battery case 14, which are in contact with each other, to each other by applying heat and pressure to the opposite sides 14a and upper and lower ends 14b and 14c of the upper and lower battery case parts of the battery case 14 so as to weld the resin layers to each other. According to circumstances, the opposite sides 14a and upper and lower ends 14b and 14c of the upper and lower battery case parts of the battery case 14 may be bonded to each other using a bonding agent. For the opposite sides 14a of the battery case 14, the same resin layers of the upper and lower battery case parts of the battery case 14 are in direct contact with each other, whereby uniform sealing at the opposite sides 14a of the battery case 14 is accomplished by welding. For the upper and lower ends 14b and 14c of the battery case 14, on the other hand, the electrode leads 11 and 12 protrude from the upper and lower ends 14b and 14c of the battery case 14. For this reason, the upper and lower ends 14b and 14c of the upper and lower battery case parts of the battery case 14 are thermally welded to each other, while a film-shaped sealing member 16 is interposed between the electrode leads 11 and 12 and the battery case 14, in consideration of the thickness of the electrode leads 11 and 12 and the difference in material between the electrode leads 11 and 12 and the battery case 14, so as to increase sealability.
In the pouch-shaped battery 10, however, the battery cell 13 repeatedly expands and contracts during the charge and the discharge of the pouch-shaped battery 10. As a result, the thermally welded regions of the upper and lower ends 14b and 14c, especially the opposite sides 14a, of the upper and lower battery case parts of the battery case 14, are easily separated from each other. Furthermore, the mechanical strength of the battery case 14 is low. In order to solve this problem, there have been proposed a method of applying an epoxy resin or a silicon resin to the outside sealing region of the battery case 14, and a method of mounting batteries in additional members having sufficient mechanical strength, respectively, and stacking the additional members one on another.
However, the above-mentioned methods do not provide high sealing force. Furthermore, when the above-mentioned methods are used, the total weight and the total size of the battery pack are increased, and the assembly process of the battery pack is very complicated.
In order to solve the above-mentioned problems, there has been proposed a secondary battery comprising an electrode assembly for charging and discharging mounted in a battery case including a metal layer and a resin layer, wherein a molding part having a predetermined thickness is at least partially formed at the outside of the battery case, which is disclosed in Korean Patent Application No. 2005-0047765, which has been filed in the name of the applicant of the present patent application. When the secondary battery is a thin-type battery, such as a pouch-shaped battery, the molding part increases the mechanical strength of the battery case, and further increases the sealing force at the sealing region. Furthermore, the molding part provides a stable stacking structure when a plurality of secondary batteries are stacked one on another so as to manufacture a medium- or large-sized battery pack.
In some small-sized secondary batteries as well as the above-described secondary battery, a cap assembly, such as a protection circuit, is integrally formed at the battery cell by insert injection molding. During the insert injection molding, a molding member is formed at a part or the entirety of the battery cell. At this time, the battery cell is brought into contact with a mold. However, the battery case of the battery may be damaged due to such contact. According to circumstances, short circuits may occur.
For example, when a molding member is formed at a specific region of the battery cell, it is necessary that the remaining parts of the battery cell be brought into tight contact with the mold, whereby injection of a material for insert injection molding (e.g., a molten material) is prevented. To this end, the mold and the corresponding region of the battery cell must be brought into tight contact with each other with a high sealing force. During this procedure, however, the battery case may be damaged. The damage to the battery case causes leakage of an electrolyte from the battery cell. Especially in a medium- or large-sized battery pack, in which a plurality of unit cells are stacked one on another, this may cause a serious accident.